"There is water on the window."
Translation:Tá uisce ar an bhfuinneog.
Because in Ulster, use of preposition plus singular definite article (otherwise known as the dative) lenites, and the standard has revently changed to accept it.
Prepositions typically eclipse the following noun, and <f> is eclipsed by <bh>. You then pronounce the <bh> in place of the <f>.